Turnbull High School
Physics Department
S4 Physics
Unit 2:- Electricity and Energy
Section 5:
Heat Energy
Name:
Class:
1
Unit 2: Section 5
By the end of this section you should be able to:
1. State that the temperature of a substance is a measure of the average kinetic energy
of the particles of the substance.
2. State that the same mass of different materials needs different quantities of heat
energy to change their temperature by one degree Celsius.
3. Carry out calculations involving specific heat capacity.
4. State that heat is gained or lost by a substance when its state is changed
5. State that a change of state does not involve a change in temperature.
6. Carry out calculations involving specific latent heat.
7. Carry out calculations involving energy, work, power and the principle of conservation
of energy.
Units, prefixes and scientific notation
1.
Use SI units of all quantities appearing in the above Content Statements.
2.
Give answers to calculations to an appropriate number of significant figures.
3.
Check answers to calculations.
4.
Use prefixes (m, k, M).
5.
Use scientific notation.
2
Electricity and Energy
Section 4 Heat
Introduction
In this Section of work the effect of heat on materials will be
investigated. This will involve changes in the temperature and changes in
the state of the material. How we measure the energy needed to change
the temperature and to change the state of a material will also be
discussed.
Heat and Temperature
•
Heat, just like light and sound is a form of energy and is measured in
joules (J).
•
Temperature is a measure of how hot a substance is and is measured in
degrees Celsius (oC).
•
The temperature of a substance tells you about the average kinetic
energy of the particles of the substance.
•
The higher the temperature of a substance the greater the average
kinetic energy of the particles of the substance.
3
4.1 Heat loss and temperature difference
A pupil heated two identical blocks to 100°C and then put one into cold
water and the other into warm water.
The pupil measured the temperature of each block every minute for 10
minutes and put the results into the table shown.
Time /
minutes
0
1
2
3
4
5
6
7
8
9
10
Temperature of block in
warm water / °C
100
67
60
55
53
51
50
49
48
47
46
Temperature of block in
cold water / °C
100
47
40
38
37
36
35
34
33
32
31
Plot both sets of results on the same graph.
•
Which block shows the greater change in temperature in 10 minutes?
•
Which block lost most energy?
4
The __________
____ of heat loss depends on the ______________
______
in
temperature between an object and it’s
it surroundings.
The_______________
_____ the difference in temperature the greater the
rate of heat loss.
On different days the inside house and outside air temperatures are as
shown below.
• On which day does the house cool down
a) most?
(b) least?
• What would happen on day 6?
5
A beaker of boiling water was placed on a bench. As the water cooled
down its temperature was measured. The results are shown in the graph
below.
• What was the water temperature at the start of the experiment?
• What was the water temperature after one minute?
• What was the drop in temperature in this time?
• Does the temperature drop by the same amount every minute?
• What do you think would be the final temperature of the water?
• How do you explain this?
6
4.2 Conduction, Convection and Radiation
There are three methods
of heat transfer called
conduction, convection and
radiation. These three
methods cause heat to be
transferred from a hot
object to the cooler
surroundings.
Conduction
Heat (energy) is transferred through a material from those parts at high
temperatures to those at low temperatures. The material does not move.
Metals are better conductors than non-metals.
Liquids and gases are poor conductors (good insulators).
Convection
Heat is transferred by the actual movement of the heated particles.
Convection can only take place in fluids i.e. liquids and gases.
The heated fluid becomes less dense and rises, cold fluid falls to take its
place i.e. convection currents are set up.
Radiation
Energy travels directly from the hot source to the object through the
space between and does not need particles.
Radiation can travel through a vacuum, this is the way in which the Earth
is heated by the Sun.
Dull/dark surfaces are better radiators of heat than polished/light
surfaces.
Dull/dark surfaces are better absorbers of heat than polished/light
surfaces.
Insulators
Materials such as cotton wool, felt, woollen clothes, etc., are good
insulators. This is because they contain trapped air which cannot move so
no convection takes place. Air is also a non-metal so it is a poor conductor
of heat.
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4.3 Heat Loss and Heat Storage
The diagram below shows how heat energy escapes from a house.
The energy losses can be reduced by insulating the house.
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4.4 Specific Heat Capacity
The energy needed to change the temperature of a substance depends on:
• the change in temperature (∆T);
• the mass of the material (m);
• the type of material (specific heat capacity, c).
Your teacher will perform a set of experiments using Virtual Physics
The specific heat capacity of a substance is the amount of energy
required to change the temperature of 1 kg of a substance by 1°°C.
The units of specific heat capacity are joules per kilogram per degree
Celsius (J kg−1 °C−1).
Energy required to change temperature
= specific heat capacity x mass x change in temperature
Eh = c x m x ∆T
Selected Materials
Specific Heat Capacities (JKg-1oC-1)
Water
Alcohol
Oil
Iron
Aluminium
Copper
4180
2350
2130
480
902
386
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Example
How much energy is needed to heat 0.5 kg of water from 20°C to 60°C?
The specific heat capacity of water is 4180 J kg−1 °C−1.
Eh = ?
c = 4180 J kg−1 °C−1
m = 0.5 kg
∆T = 60 − 20 = 40°C
Eh = c m ∆T
= 4180 x 0.5 x 40
= 83 600 J
Note:
1. When a solid, liquid or gas is heated, the particles begin to move more
violently and so the average kinetic energy of the particles increases
- this is observed as an increase in temperature of the material.
As the average Ek of the particles increases, the temperature of the
material increases.
2. Specific heat capacity is the energy, which causes the average Ek of the
particles of a material to change and so causes a change in temperature.
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Tutorial 1
1. Calculate the amount of heat energy required to raise the temperature
0.5kg of water from 18oC to 58oC.
2. Calculate the amount of heat energy required to raise the temperature of
0.95kg of iron from 18oC to 198oC.
3. How much energy does 5 kg of steel give out as it cools from 35°C to
20°C? The specific heat capacity of steel is 500 J kg−1 °C−1.
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4.5 Heat Problems
Your teacher will perform an experiment using Virtual Physics
The Principle of Conservation of Energy
Although energy can be changed from one form to another, the total
amount remains unchanged.
Due to conduction, convection and radiation, some of the energy supplied
will be transferred ("lost") to the surroundings.
This means that the substance will absorb (take in) less energy than was
supplied by the heater.
Energy supplied by heater =
Energy absorbed by material + Energy ‘lost’ to surroundings
In most heat problems, it is assumed that no energy is transferred to the
surroundings, hence
Energy supplied by heater = Energy absorbed by the material
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Example
An immersion heater has a power rating of 200 W. It is used to heat 0.2
kg of a material from 20°C to 65°C. The time taken to do this is 3
minutes. Calculate the specific heat capacity of the substance?
P = 200 W
c=?
t = 3 minutes = 3 x 60 = 180 s
m= 0.2 kg
∆T= 65 − 20 = 45°C
Energy supplied = Energy supplied by heater
= Power x time
= 200 x (3 x 60)
= 36 000 J
Energy absorbed = Energy needed to change temperature of substance
= cm∆T
= c x 0.2 x 45
=cx9
If no energy transferred to the surroundings:
Energy absorbed = Energy supplied
c x 9 = 36 000
c = 36 000 = 4000 J kg−1 °C−1
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Specific heat capacity of substance is 4000 J kg−1 °C−1
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Tutorial
1. The p.d. across a heater is 230 V. The heater heats 0.3 kg of water for
two minutes. The temperature of the water rises by 30°C. Calculate the
current in the element of the heater.
(Specific heat capacity of water = 4180 J kg−1 °C−1)
2. An insulated beaker contains 100g of water at a temperature of 80°C.
200 g of water at a temperature of 70°C is poured into the insulated
beaker. Calculate the temperature of the resulting mixture assuming that
no energy is transferred with the surroundings.
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4.6 Cooling Curves
Your teacher will show you an experiment involving test-tubes containing
equal amounts of water and a substance called salol. Both test-tubes have
been heated to approximately 50°C.
Time (minutes)
Temp water (°C)
Temp X (°C)
Time (minutes)
Temp water (°C)
Temp X (°C)
• Plot a graph of temperature against time for both liquids, on the same
graph (in different colours) – these are known as cooling curves.
• Does the water change state?
• Does substance X change state?
• Did the temperature of the water remain constant for a time?
• Did the temperature of X remain constant for a time?
At all times, the water and X must have been losing (transferring) heat
because they were hotter than their surroundings.
When X was at a constant temperature, it must still have been losing heat.
Extra heat must have been produced within X to keep the temperature
constant.
• What is happening to X at this time?
When a substance is changing it’s _________ its temperature stays
_______________.
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When a substance changes from a solid to a liquid or a liquid to a
gas, energy is needed to break down the force (or bond) holding
the particles together and to push the particles further apart.
Specific Latent Heat of Fusion – the energy needed to change 1 kg from
solid to liquid without change in temperature.
Specific Latent Heat of Vaporisation – the energy needed to change 1 kg
from liquid to gas without change in temperature.
When a substance changes state from solid to liquid or liquid to gas, latent
heat is absorbed (taken in).
When a substance changes state from gas to liquid or liquid to solid, latent
heat is released (given out).
Your teacher will perform an experiment using Virtual Physics
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4.7 Using Latent Heat
Refrigerator
The inside of a refrigerator is kept
cool by changing the state of a
fluid, which has a very high latent
heat of vaporisation. When the
liquid reaches the freezer box, it
expands rapidly through a small
valve and evaporates. The heat
required to do this is taken from
the freezer box, which becomes
very cold.
The gas is pumped and compressed
through thin pipes at the back of
the refrigerator, where it releases
latent heat into the room and
changes state i.e. becomes a liquid.
Cool Boxes
Food is sealed in a very well insulated
container, with some frozen chemical packs on
top. These were cooled in a freezer until the
chemicals changed from liquid to solid.
Initially the cold frozen packs keep the box
cool, as they take heat from the food.
However, the temperature will slowly rise and
the chemicals will begin to melt i.e. change
from a solid to a liquid. This change of state
takes heat from the food which is kept cool
for a longer period.
• Why are the chemical packs always placed on top of the food?
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4.8 Specific Latent Heat of Vaporisation of Water
When a liquid is changing to a gas, latent heat is required.
This energy is called latent heat of vaporisation.
When water changes to steam, the heat required to change 1 kg of water
to 1 kg of steam, without temperature change is called the specific latent
heat of vaporisation.
4.9 Specific Latent Heat of Fusion of Water
When a solid is changing to a liquid, latent heat is required. This energy is
called latent heat of fusion.
When ice changes to water, the heat required to change 1 kg of ice to 1 kg
of water, without temperature change is called the specific latent heat of
fusion.
4.10 Specific Latent Heat
The symbol for specific latent heat is l, and it is measured in J kg−1.
Eh = ml
Energy needed to change state = mass x specific latent heat
Look at the
state of me!!
18
Tutorial 2
1. The power rating of the heating element of a kettle is 2200 W. The
kettle is used to bring 1.5 kg of water to its boiling point. The kettle is
left on for a further 30 s.
Calculate the mass of water that is boiled off in this time.
(The specific latent heat of vaporisation of water = 2 260 000 J kg−1)
2. A heater is being used to melt some ice at a temperature of 00C. The
power rating of the heater is 20 W.
Calculate the time needed to change 40 g of ice at 00C into water at 00C.
(The specific latent heat of fusion of ice = 3.34 x 105 J kg-1)
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Unit 2: Section 4 - Additional notes
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Unit 2: Section 4 - Additional notes
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Unit 2: Section 4 - Additional notes
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Unit 2: Section 4 - Additional notes
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Unit 2: Section 4 - Additional notes
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